Electric circuit breaker comprising ceramic capacitor elements connected in parallel with its contacts



3,382,412 R COMPRISING CERAMIC CAPACITOR N PARALLEL- WITH ITS CONTACTSMay 7, 1968v MANKOFF ETAL ELECTRIC CIRCUIT BREAKE ELEMENTS CONNECTED IOriginal Filed Nov. 1, 1963 F M .R M m y 0 W M M 1 m V E W MW N A ER a VR 2\// a H M V 0 0 0 r a L I ---\-k-\--.---- l I U 5 Vvy United StatesPatent Ofiice 3,382,412 Patented May 7, 1968 3,382,412 ELECTRIC CIRCUITBREAKER COMPRISING CE- RAMIC CAPACITOR ELEMENTS CONNECTED IN PARALLELWITH ITS CONTACTS Lawrence L. Mankoit, Broomall, and Roy Nakata, BrynMawr, Pa., assignors to General Electric Company, a corporation of NewYork ,Original application Nov. 13, 1963, Ser. No. 323,414, now

Patent No. 3,325,708, dated June 13, 1967. Divided and this applicationMay 22, 1967, Ser. No. 640,033

3 Claims. (Cl. 317-58) ABSTRACT OF THE DISCLQSURE This application is adivision of application S.N. 323,- 414, filed Nov. 13, 1963, now PatentNo. 3,325,708.

This invention relates to an electric circuit breaker that comprises apair of separable contacts and a capacitor assembly comprisingseries-connected ceramic capacitor elements connected in parallel withthe contacts.

When the contacts of an energized circuit breaker of this type aredriven from open to closed position, a short circuit path is establishedthrough the contacts across the terminals of the capacitor assembly, andthe capacitor is free to quickly discharge through this path. It hasbeen found that this sudden discharge subjects the ceramic capacitorelements to high mechanical stresses which sometimes even crack theceramic. The mechanical stresses are believed to be due to anelectrostrictive etiect, the severity of which varies directly with thepeak voltage appearing across the capacitor following initial capacitordischarge.

An object of our invention is to prevent damage to the capacitorelements from electrostrictive effects resulting from rapid discharge ofthe capacitor assembly.

For a better understanding of our invention, reference may be had to thefollowing description taken in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a side elevational view, mostly in section, showing acapacitor assembly embodying one form of our invention.

FIG. 2 is a perspective View, partly in section, showing a component ofthe assembly of FIG. 1.

FIG. 3 is a schematic view of an electric circuit breaker containing thecapacitor assembly of FIG. 1.

Referring now to FIG. 1, the capacitor assembly is designated 9 andcomprises a plurality of ceramic capacitor elements 10 stacked inend-to-end relationship along the centrally-located longitudinal axis 11of the capacitor assembly. In the illustrated embodiment, each capacitorelement 10 is a cylindrical disk having a pair of opposed planar faces12 extending transversely of the longitudinal axis 11 and a periphery 14extending between the planar faces 12 generally parallel to thelongitudinal axis 11. A perspective view of one of the ceramic elements10 is shown in FIG. 2. Each of the ceramic elements is made of a ceramicmaterial having a high dielectric strength and a high dielectricconstant, such as, for example, barium titanate. The opposed faces 12 ofeach ceramic element are covered by a thin coating 15 of conductivematerial bonded thereto. Preferably, this coating is a silver dispersionfused integrally onto the ceramic face 12. This silver coating coversthe whole face 12 of the ceramic element so that its outer periphery islocated at the outer periphery of the ceramic element.

For connecting the ceramic capacitor units in series circuitrelationship, a plurality of conductive buttons 18, preferably of steelcadmium plated to resist corrosion, are provided. One of theseconductive buttons 18 is located between each adjacent pair of ceramiccapacitor units 10. Each of these conductive buttons is preferably of acylindrical form and has a pair of opposed planar surfaces Which contactthe silver coatings 15 of the ceramic units 10 to provide a goodelectrical connection between the button and the silver coatings. It isimportant to note that these buttons 18 simply bear against adjacentsilver coatings and are not attached to them.

For maintaining the ceramic elements 10 and the buttons 18 in superposedrelationship as shown in FIG. 1, a tube 24 of insulating material isprovided about these elements. At opposite ends of the insulating tube24, metallic terminal structure 26 and 28 for the capacitor assembly areprovided. These terminal structures 26 and 28 are respectively connectedto the insulating tube 24 by suitable means such as threaded joints 29and 30. These terminal structures 26 and 28 are connected in a highvoltage power circuit by suitable means (not shown). At the top of thecapacitor assembly, a conductive button 32 brazed to the top terminalstructure 28 bears against the silver coating 15 on the top of the topceramic elemeat 10. At the bottom of the capacitor assembly, aconductive button 34 bears against the silver coating on the bottom ofthe bottom capacitor element 10. A flexible conductive connection in theform of a compression spring 36 is provided between the lower terminalstructure 26 and the button 34 to carry current between these parts.This spring 36 bears at one end on the terminal structure 2 6 and at itsother end on the button 34. Contact between the buttons and theiradjacent capacitor elements is maintained by the compression spring 36.This compression spring 36 provides an upward force which urges eachelement of the assembly upwardly into firm engagement with its adjacentelement. It will therefore be apparent that the capacitor elements 10are electrically connected together in series circuit relationshipbetween the terminal structures 26 and 28.

The ceramic elements 10 are of a substantially larger diameter than theconductive buttons 18. As a result the silver coating 15 on each ceramicelement projects radially outward considerably beyond the outerperiphery of its adjacent button 18 and has an outer peripheral portionextending around the outer periphery of the button. When a high voltageis applied between these silver coatings on opposite faces of theceramic element, high electrical stress concentrations tend to occur atthe outer periphery of the silver coatings. If these high electricalstress concentrations occurred in air, they could ionize the air, andthis could lead to a damaging flashover along the periphery of theceramic element between the silver coatings. We preclude such ionizationand resultant fiashover by covering the periphery of the silver coatings15 and the periphery of the ceramic element 10 with a continuous coating40 of high dielectric strength insulating material. This insulatingcoating 40, which is preferably made of an epoxy resin, is bonded to theceramic element and the silver coatings along the entire interfacebetween the insulating coating 40 and the ceramic element 10 and silvercoatings 15. This bond prevents any air from being trapped in thisinterface region. Thus, the high electrical stresses that occur at theperiphery of the silver coating are located in the solid insulatingmaterial of coating 40, which is able to withstand the stresses withoutbreakdown. In a preferred form of our invention, a thin layer of varnishis disposed between the epoxy coating and the adjacent parts to providefor an improved bond between the epoxy coating and these adjacent parts.

The insulating coating 49 of each ceramic element serves the additionalfunction of positioning the conductive button 18 and limiting the extentto which the button can shift transversely of the assembly. In thisrespect, note that the insulating coating 46 does not cover the centralregion of the ceramic element 16 where the button is located. Theradially-inwardly projecting flanges 41 that constitute a part of theinsulating coating 40 terminate short of the button 18. The innerperiphery of the flanges 41 of the insulating coating 49 thus forms anannular shoulder 42 having a diameter slightly greater than that of thebutton 18. This shoulder 42 is capable of limiting lateral shifting ofthe button 18, thus helping to assure that the capacitor elements aremaintained in their desired assembled relationship. It is noted that theinsulating coating 40 is unattached to the button 18.

Since there is no bond between each of our conductive buttons 18 and itsadjacent silver-coated ceramic element, it will be apparent that each ofthese parts can expand and contract freely in response to temperaturevariations Without interference from the adjacent part and withoutimpairing the electrical connections between these parts. Unequalexpansion or contraction simply results in a slight sliding of onemetallic surface over the other in a direction laterally of the assembly9. But despite this slight sliding, the parts are maintained in goodelectrical connection by the spring 36.

Since there is no bond between the insulating coating 40 and the button18 that it surrounds, the flanges 41 of the insulating coating are freeto contract and expand along the face 12 of the ceramic element withoutimposing any lateral or other forces on the button 18. This feature,together with the absence of attachment between the button 18 and itsceramic element 1!), enables the insulating material 40 to expand andcontract along the face 12 without detrimentally affecting the portionof the silver coating 15 that the button 18 bears against and withoutdetrimentally affecting the bond between this portion of silver coating15 and the ceramic body 10.

It has been found that if the button 18 is soldered to the silvercoating 15 and the insulating coating 40 is bonded to the button, thesoldered joint and the adjacent ceramic can be damaged when theinsulation 40 contracts in response to a temperature drop. Suchcontraction tends to create forces which act radially-outward on theflanges 41 of the insualting coating, as indicated by arrows 43. If thebutton 18 Were bonded to the insulation 41) about the button periphery,these radially-outward forces would severely stress any joint presentbetween the button 18 and the ceramic element 10. This effect ismagnified if there is an uneven bond between the insulation 46 and thebutton about the button periphery.

When the insulation 49 contracts as indicated by the arrows 48, it doesimpose shear stresses on the joint between the silver coating and theceramic, but these stresses are not high and can be readily withstood bythis joint. These stresses can be reduced even further by reducing theextent to which insulating flanges 41 extend radially inward from theouter periphery of the silver coating 15. In a preferred form of ourinvention, each of these flanges 41 extends radially-inward along only aminor portion of the radius of the silver coating 15, this radius beingmeasured from the central axis 11 to the outer periphery of the silvercoating.

Our assembly is highly compact because the metallic buttons 18 can berelatively thin, as considered along the longitudinal axis 11 of theassembly. This thinness does not interfere with the main function of thebuttons, i.e., to electrically interconnect the adjacent capacitorelements. All that is needed in the way of thickness for the buttons issuflicient thickness to assure that the insulating coatings 40 onadjacent ceramic elements will not engage under the pressure of spring36. Such engagement between the insulating coatings 40 could interferewith good engagement being obtained between the buttons 18 and theconductive coatings 15 on the ceramic elements.

A typical application in which capacitor assembly of our invention isutilized is an air blast circuit breaker, such as the circuit breakerdisclosed and claimed in US. Patent 2,783,338-Beatty, assigned to theassignee of the present invention. Such a circuit breaker isschematically depicted in FIG. 3. This circuit breaker of FIG. 3comprises a metallic tank 50 filled with pressurized air at a relativelyhigh pressure, for example, 500 psi. The tank 50 is at a high voltageand is isolated from ground by a suitable insulator 52 on which it issupported. Disposed within the tank 50 are two pairs of separablecontacts 54 and 56 connected together in series circuit relationshipbetween the terminals 58 and 60 of the circuit breaker. Each of thesepairs of cont-acts comprises a movable contact 59 which is controlled byan operating mechanism 59a. The operating mechanism 59a operates the twomovable contacts in unison, simultaneously closing the contacts andsimultaneously opening them. A preferred form of mechanism is shown andclaimed in the above-mentioned Beatty patent.

The set of contacts 54 comprises a stationary contact 61 that is mountedat the inner end of a conductive stud 62. The other set of contacts 56comprises a stationary contact 63 that is mounted at the inner end of aconductive stud 64. The two conductive studs 62 and 64 are respectivelysupported on the tank 50 by insulating bushings 65, which electricallyinsulate the studs from the tank when the contacts are open. Theconnection between the two sets of contacts is electrically connected tothe tank by a conductor 67.

The capacitor assembly 9 of FIG. 1 is schematically illustrated in FIG.3 connected between the conductive stud 62 and the tank 511 so that itis in parallel with the break 54. An identical capacitor assemblydesignated 9a is electrically connected between the other conductivestud 64 and the tank 50, so that it is in parallel with the other break56. One of the purposes of these capacitor assemblies in the circuitbreaker of FIG. 3 is to divide the voltage that is applied between theterminals 58 and 60 substantially equally between the twoseries-connected breaks 54 and 56.

When the circuit breaker of FIG. 3 is operated to open position, a blastvalve (not shown) is opened to create a blast of air for aiding in thecircuit interrupting operation. This opening of the blast valve duringinterruption results in a sudden drop in pressure inside the tank 59. Itis important that this pressure drop not create pressure differentialsin the capacitor assembly 9 that could, even temporarily, move theelements of the capacitor assembly out of engagement in opposition tothe spring pressure exerted thereon. To prevent such pressuredifferentials from developing, we provide the tube 24 with a largenumber of openings which afford free communication between the spaceinside the tube and the space in tank 50 that is outside the tube.Preferably, these openings are provided by making the tube 24 of a looseweave fiber glass construction that has relatively large openingsbetween the strands that make up the tube. This tube construction isdepicted in FIG. 1, where the openings are designated 70. A suitablerigid plastic coats the fibers to impart rigidity to the tube but doesnot fill the above described openings in the tube. With these openingspresent, a pressure change outside the tube 24 results in acorresponding change throughout the entire interior of the tube. Thus,no pressure differentials are developed inside the tube that could movethe elements of the capacitor assembly out of engagement.

Assume now that the circuit through the circuit breaker of FIG. 3 isenergized an that the movable contacts 59 are driven into closedposition. When the contacts reach the closed position, a short circuitpath is established through the contacts across the terminals of eachcapacitor assembly 9 or 9a, and the capacitor assembly is free toquickly discharge through this short circuit path. The short circuitpath that is present across the capacitor assembly 9 extends through thetank 50, conductor 67 and contacts 54.

It has been found that this sudden discharge subjects the ceramiccapacitor elements It} to high mechanical stresses which sometimes evencrack the ceramic. These mechanical stresses are believed to be due toan electrostrictive effect, the severity of which varies directly withthe peak voltage appearing across the capacitor assembly when thecircuit oscillates following initial capacitor discharge. In oneembodiment of our invention, we limit this peak voltage by providingenough resistance in circuit with the capacitor assembly to over-dampthe capacitor discharge circuit, thus preventing any substantialovershoot of the voltage when the capacitor initially discharges. Weprovide this resistance by making the buttons 18 of a material that hasenough resistance to impart the desired overdamping. This overdampingenables us to limit the peak voltage across the capacitor assembly to avalue of one half or less that which would otherwise be developed, thussubstantially reducing the clectrostrictive effect on the ceramicelements.

Although the maximum amount of resistance present in the capacitorassembly is not highly critical, this maximum should be sufiiciently lowas to preclude overheating of the capacitor assembly by currents flowingtherethrough when the contacts 54, 56 of the circuit breaker are opened.

The insulating coating 40 on each ceramic element serves an additionalfunction not mentioned hereinabove. This function is that of acting asan energy damper that reduces the tendency of the ceramic element tocrack in response to the electrostriction of the ceramic that resultsfrom sudden capacitor discharge.

In certain circuit breakers, there will be a resistor that is connectedacross each break just prior to the instant at which the break isclosed. This resistor is a further help in reducing the amplitude of thevoltage peak that will appear across the capacitor assembly when thebreak is closed.

While We have shown and described particular embodiments of ourinvention, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from ourinvention in its broader aspects, and we, therefore, intend in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In an electric circuitibreaker comprising a pair of separablecontacts,

(a) a capacitor assembly having a centrally-located longitudinal axisand comprising:

(i) a plurality of ceramic elements, each having a pair of opposed facesdisposed transversely of said longitudinal axis and an outer peripheryextending between said faces,

(ii) a conductive coating bonded to each face of the ceramic elementsand having an outer periphery disposed at the outer periphery of theassociated ceramic element,

(iii) the conductive coatings on opposed faces of each ceramic elementbeing electrically insulated from each other by said ceramic element,

(iv) means for mounting said ceramic elements in end-to-end relationshipalong said longitudinal axis,

(v) a plurality of conductive buttons respectively mounted betweenadjacent pairs of said ceramic elements but unattached to said ceramicelements,

(vi) each of said buttons having opposed surfaces that engage theconductive coatings on adjacent faces of adjacent ceramic elements,

(vii) said ceramic elements and the conductive coatings thereonextending transversely of said longitudinal axis to a greater extentthan said buttons so that a portion of said conductive coating islocated transversely beyond the periphery of the adjacent button allaround the button periphery,

(viii) a coating of insulating material on each face of said ceramicelements covering the outer periphery of said conductive coating andbonded to said conductive coating, said coating of insulating materialcovering the surface of said conductive coating from the outermostperiphery to a location spaced radially inwardly from said outermostperiphery,

(ix) said coating of insulating material having an inner peripheryextending about the adjacent button and forming a shoulder adapted tolimit transverse shifting of said button, said button and saidinsulating coating being unattached to each other,

(x) and spring means at one end of said assembly for exerting a forcealong said longitudinal axis that maintains said buttons in contact withthe conductive coatings on adjacent ceramic elements,

(b) means for connecting said capacitor assembly in parallel with saidcontacts,

(c) said conductive buttons having sufficient resistance to overdamp thecircuit comprising said capacitor assembly and said contacts when saidcontacts are closed while said circuit is energized.

2. An electric circuit breaker comprising:

(a) a plurality of pairs of separable contacts adapted to be connectedin series in a power circuit,

(b) means for controlling the voltage distribution between said pairs ofcontacts when said circuit breaker is open comprising a plurality ofcapacitor assemblies respectively connected across said pairs ofcontacts,

(c) each of said capacitor assemblies comprising a plurality of ceramicelements mounted in end-to-end relationship,

(d) closing of each of said sets of contacts establishing a dischargecircuit through said set of contacts for the capacitor assemblyconnected thereacross.

(e) resistance means in each of said discharge circuits connected inseries with said ceramic elements and said contacts,

(f) said resistance means having a sufficiently high resistance tooverdamp said discharge circuit when said contacts are closed while saidpower circuit is energized.

3. The electric circuit breaker of claim 2 in which each of saidcapacitor assemblies comp-rises conductive but tons of substantialresistance interposed between adjacent ceramic elements, said resistancemeans of claim 2 being constituted by said buttons.

References Cited UNITED STATES PATENTS 2,572,598 10/1951 Curtis 3l7112,758,254 8/1956 Kramer 317-11 3,325,708 6/1967 Mankofi et al. 31726lMILTON O. HIRSCHFIELD, Primary Examiner.

R. V. LUPO, Assistant Examiner.

